This invention relates to magnetic flowmeters, and more particularly to a reference-voltage detecting circuit in a magnetic flowmeter whose electromagnet is excited by a low-frequency excitation current.
In order to eliminate unwanted fluctuations from the flow rate signal yielded by a magnetic flowmeter, which fluctuations result from fluctuations in the excitation current for the electromagnet, the general practice is to provide a reference-voltage detecting circuit. This circuit is arranged to produce a reference-voltage proportional to the excitation current, the ratio of this reference-voltage to the flow rate signal being determined by means of a divider.
The reference-voltage is usually derived from the secondary winding of a reference-voltage detecting transformer whose primary winding is interposed in series with the excitation circuit of the electromagnet. This is done in order to isolate the divider from the commercial power line utilized as the source for the excitation current.
When an a-c current having the usual 50 or 60 Hz commercial power line frequency serves as an excitation current source, one has no difficulty in obtaining the reference-voltage from the secondary winding of the reference transformer.
Also known are magnetic flowmeters which make use of a low-frequency excitation wave, such as one whose excitation frequency is well below that of the commercial power line frequency. One can, by means of a low-frequency excitation type magnetic flowmeter, obtain a flow rate signal having an excellent signal-to-noise ratio; for unwanted magnetic coupling and/or electrostatic coupling between the excitation coil and the signal lead wires connected to the flow tube electrodes may be reduced considerably.
A low-frequency excitation type magnetic flowmeter also requires an arrangement for eliminating fluctuations in the flow rate signal resulting from fluctuations in the excitation current. And to this end, a transformer is included in the circuit in order to detect a reference voltage proportional to the excitation current.
When a low-frequency excitation current is produced by chopping the d-c output of a d-c voltage source by an "on-off" controlled switching means, a d-c component is present in the chopped excitation current as the mean value thereof. As a consequence, the magnetic core of the transformer serving to detect the reference voltage is magnetically saturated by this d-c component. Because of such core saturation, the reference voltage derived from the secondary winding of the transformer is distorted and the adverse effect of excitation current fluctuations cannot be fully removed from the flow rate signal.
To overcome this drawback, one could conceivably employ a transformer whose core is so large that it is not subject to magnetic saturation by the d-c component of the excitation current. However, this solution creates another problem; for the size of the reference-voltage detecting circuit then becomes excessively bulky and cumbersome.
Yet another technique for eliminating distortion resulting from the d-c component of the excitation current is by means of switching elements adapted to apply the excitation current in both directions from positive and negative d-c sources. The disadvantage of this technique is that the resultant configuration of the excitation current circuit becomes relatively complex.